Communication devices such as User Equipments (UE) are enabled to communicate wirelessly in a wireless communications system, sometimes also referred to as a cellular radio system or a cellular network. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between a user equipment and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the wireless communications system.
User equipments are also known as e.g. mobile terminals, wireless terminals and/or mobile stations, and may further be referred to as mobile telephones, cellular telephones, or laptops with wireless capability, just to mention some further examples. The user equipments in the present context may be, for example, portable, pocket storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity.
The wireless communications system covers a geographical area which is divided into cell areas, wherein each cell area, being served by a network node such as a Base Station (BS), e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. eNB, eNodeB, NodeB, B node, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pica base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several radio access and communication technologies. The base stations communicate over the radio interface operating on radio frequencies with the user equipments within range of the base stations.
In some RANs, several base stations may be connected, e.g. by landlines or microwave, to a radio network controller, e.g. a Radio Network Controller (RNC) in Universal Mobile Telecommunications System (UMTS), and/or to each other. The radio network controller, also sometimes termed a Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural base stations connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Group Special Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
UMTS is a third generation mobile communication system, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipments. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
According to 3GPP/GERAN, a user equipment has a multi-slot class, which determines the maximum transfer rate in the uplink and downlink direction. GERAN is an abbreviation for GSM EDGE Radio Access Network. EDGE is further an abbreviation for Enhanced Data rates for GSM Evolution.
In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
When a user equipment is powered on, it starts the initial cell selection procedure. The purpose of the initial cell selection procedure is to ensure that the user equipment gets into service as fast as possible. The user equipment uses this procedure to scan all Radio Frequency (RF) channels in the E-UTRA bands according to its capabilities to find a suitable cell, where suitable refers to a combination of radio measurements based criteria, e.g. a Reference Signal Received Power (RSRP) and other criteria, e.g. the cell belonging to a Public Land Mobile Network (PLMN) that the user equipment is allowed to camp on.
Once the user equipment camps on a suitable cell, it continuously searches for the best cell to camp on. This procedure is called cell reselection. Its purpose is to ensure that the user equipment always camps on the cell that is best in terms of some predetermined set of criteria celled the cell ranking criteria. Broadly speaking, cell reselection is the basic idle mode mobility procedure that ensures that the user equipment continuously may receive system broadcast information and continuously may be paged within the coverage area of its RUM. Cell reselection also ensures that once paged, the user equipment will enter connected mode in a cell that provides good coverage.
Handover refers to the transfer of an ongoing call or data session from one channel connected to a core network to another channel.
Cell change may be used to describe the mobility when the user equipment performs inter-radio access technology (inter-RAT) handovers, e.g. between a GERAN and a WCDMA network. In some networks, such an inter-RAT cell change is supported by sending system information of the new network while still connected to the old network and thereby facilitating the inter-RAT mobility procedure. Cell change may also refer to the collection of idle and connected mode mobility, such as cell (re-)selection and handover procedures, but it may also include the cases of inter-RAT and inter-frequency mobility.
In network assisted device-to-device (D2D) communications underlaying a cellular infrastructure, user equipments in the proximity of each other may establish a direct radio link, also referred to as a device-to-device link or bearer. In this description, the expression device-to-device will be referred to as D2D. Network assisted D2D communications is sometimes also referred to as network D2D communications underlaying a cellular radio access network, or for short D2D as a RAN underlay.
FIG. 1 schematically illustrates a network assisted D2D communications scenario in a RAN according to prior art. A user equipment A, UE-A, is connected to a first network node, e.g. a serving base station eNB-A, and a user equipment B, UE-B, is connected to a second network node, e.g. a serving base station eNB-B. The RAN comprises in this example the user equipments A, B; and the first and second network nodes, eNB-A, eNB-B. Further, the user equipment A and the user equipment B are in the proximity of each other. In such a case, the RAN may assist the user equipments A, and B to establish a D2D link over which the user equipments A, and B can communicate.
When used in this description, by user equipments in the proximity of each other is meant user equipments that are positioning within a distance of 100-150 m from each other. However, devices communicating in licensed spectrum bands, and consequently allowed to transmit with higher power levels, such as 24 dBm, may, under certain propagation conditions, be able to discover and transfer data to each other over larger distances, e.g. over distances up to 1000 m.
While the user equipments A, and B communicate over the D2D link, they also maintain a cellular connection with their respective serving base station eNB-A, eNB-B, cf. FIG. 1. That way the cellular RAN can assist the user equipments A, and B in allocating cellular resources for the D2D link, indicating a maximum power level to the transmitting user equipment and, in general protecting the D2D link from interference from cellular user equipments or base stations. The basic rationale for network assisted D2D communications is to allow short range direct communication between user equipments utilizing cellular spectrum.
Network assisted D2D communications requires that the RAN, and essentially a network node. i.e., a base station or a radio network controller, where applicable, comprised in the network, implements functionalities specifically for the purpose of supporting D2D peer discovery and D2D link establishment. Such functionality is expected to be introduced gradually in the network nodes, as the details and protocols of D2D communications mature and get standardized. For example, a new functionality specific to D2D communication is the function of mode selection The mode selection function decides whether the user equipments should communicate via the cellular infrastructure or using the direct D2D link. In general, the set of new functionalities are related to the management of interference between an D2D layer and a cellular layer, and also to manage the mobility, security and quality of service issues related to the coexistence of cellular and D2D links within the coverage area of the RAN.
A basic requirement for network nodes supporting D2D communications is that they should be able to coexist with legacy RAN nodes as the D2D functionality will be gradually introduced into the existing RAN infrastructure. By legacy RAN nodes, when used herein, is meant RAN nodes that do not support D2D communication. Likewise, legacy user equipments, i.e. user equipments not supporting D2D communication, are expected to coexist with D2D capable user equipments in radio access networks. Thus, in future RANs, network nodes and/or user equipments with or without D2D functionality are expected to be present.
FIG. 2 schematically illustrates a communications network comprising network nodes with or without D2D functionality, also referred to as a mixed communications network. The mixed communications network comprises a user equipment A, UE-A, and a D2D capable network node eNB-A to which the user equipment A is connected. The communications network further comprises a user equipment B, UE-B, and a legacy or limited D2D capable network node eNB-B to which the user equipment B, UE-B, is connected. In such a mixed communications network, D2D capable user equipments, such as user equipment A and user equipment B, in the proximity of each other cannot establish a D2D link in operator spectrum unless both user equipments A, B are connected to a D2D capable network node, such as the D2D capable network node eNB-A. By operator spectrum is meant a spectrum licensed by an operator. This forbids unauthorized access to radio resources unless permitted by operator owning the spectrum.
Thus, in a RAN that comprises heterogeneous network nodes in terms of their capabilities of supporting D2D communications, cf. FIG. 2, two D2D capable user equipments in the proximity of each other cannot establish a D2D link unless both network nodes are capable of supporting network assisted D2D communications or unless the two D2D capable user equipments are connected to the same network node capable of supporting network assisted D2D communications.
Assuming tight reuse deployments, the reason that the two D2D capable user equipments A and B in the proximity of each other cannot establish a D2D link unless both are served by an D2D capable network node is that D2D communications in the operator spectrum need to be managed such that it coexists with the user equipments A and B communicating with their respective serving network nodes, e.g., base stations. That is D2D communications in the operator spectrum need to be managed such that the D2D communications may coexist with traditional cellular user equipments communicating with their respective serving base stations in cellular mode. In other words, since the D2D layer, i.e. the underlay layer, and the cellular layer need to be managed across multiple network nodes, the management of the D2D communications and the cellular layer becomes a major problem in heterogeneous communications networks comprising both D2D capable network nodes and non-D2D capable network nodes.
When used in this description, by tight reuse deployments is meant that the frequency reuse factor between cells is close to one or equals to one. In other words, most or all of the frequency resources, such as OFDM subcarriers, are reused for communications in every cell of the cellular network. Some examples of systems with tight frequency reuse deployments are WCDMA, LTE, CDMA2000 etc.
This basic problem arises in different situations within the scenario of FIG. 2.
In the first situation, the user equipment A, UE-A, and the user equipment B, UE-B, discover the proximity of each other, but at least one of them, and maybe both, is currently not connected to, i.e. served by, a D2D capable network node, e.g., base station such as eNode B-A, eNB-A.
In the second situation, the user equipment A, UE-A, and the user equipment B UE-B, are currently connected to the same network node, e.g., base station such as eNode B-B, eNB-B, which is not D2D capable.
In the third situation, the user equipment A. UE-A, and the user equipment B, UE-B, are currently connected to the same network node, e.g., base station such as eNode B-A, eNB-A, that is D2D capable, but the user equipment A, UE-A, is about to hand over to a non D2D capable network node, e.g., base station such as eNode B-B, eNB-B.
US 2010/0261469 A1, US 2010/0009675 A1, and WO 2010/006650 A1, relate to D2D communications in general and suffer from the drawbacks with a mixed communications network described above.